1. Field of the Invention
The invention relates generally to an exhalation unit for a respirator. In one aspect, the invention relates to an exhalation unit comprising two valves having different cracking pressures. In another aspect, the invention relates to an exhalation unit comprising two valves, and the cracking pressure for the valves can be adjusted by adjusting the relative position of the two valves.
2. Description of the Related Art
Respirators for purifying ambient air and for providing a breathable air supply to a wearer are well-known devices that are utilized by firefighters, military personnel, and in other settings where individuals can potentially be exposed to a contaminated air supply. Such respirators can include masks and/or face shields for securing the respirator to the wearer's face and for further protecting the wearer. Because respirators are used in diverse environments having a wide range of air contaminants and concentrations thereof, there are multiple varieties of respirators that offer differing levels of protection.
For example, in a negative pressure respirator, which is the simplest type of respirator, the air pressure inside the mask is negative during inhalation with respect to the ambient pressure outside the respirator. As the user inhales, air is drawn from the ambient atmosphere, through an air purifying filter, and into the mask. The user then exhales through an exhalation unit typically comprising a check valve that provides a relatively small exhalation resistance. Such respirators are sufficient for certain environments, but can be susceptible to contamination if any leaks develop in the respirator or between the mask and the wearer.
A higher level of protection is provided by a powered air purifying respirator (PAPR), wherein the air pressure inside the mask is slightly positive during inhalation with respect to the ambient pressure outside the respirator. In this type of respirator, the filter attaches to a canister with a fan or blower, preferably battery operated, that forces air through the filter, and then the purified air with positive pressure runs through a hose to the mask. The exhalation resistance of the check valve in the exhalation unit can be higher than in a negative pressure respirator.
A third type of respirator system is a self-contained breathing apparatus (SCBA), which includes an air tank that is usually worn on a user's back and contains compressed purified air. The tank provides positive pressure air to the mask through a pressure reducing valve to step down the air pressure to an acceptable level. Air enters the mask through a demand valve that opens when the user inhales. Logically, the cracking pressure of the exhalation unit check valve used with the SCBA system is greater than that for use in the PAPR system and is greater than the cracking pressure of the demand valve to prevent continuous flow of air through the respirator. In this way, air flows into the respirator during inhalation but ceases to flow during exhalation. Although the supply of air in the SCBA is limited by the volume of the tank, the SCBA respirator system is portable and highly effective in environments where the air is highly contaminated and dangerous, such as in firefighting.
Alternatively, the respirator can be utilized as a closed circuit breathing apparatus (CCBA), wherein an exhale hose is attached at one end to the exhalation unit and at the opposite end to the respirator inlet connection. Hence, the respirator and the exhale hose form a closed breathing loop. During use, the user exhales through the exhalation unit, through the air purification means, and back into the respirator via the inhalation hose of the CCBA circuit.
When selecting a respirator, the user determines which type of respirator is most suitable for the intended application and environment. However, if the user wants to be prepared for multiple types of environments, will be in an environment wherein the air contamination is variable, or is not able to accurately predict the type of environment in which the respirator will be used, the user must carry multiple types of respirators, which can be bulky and inconvenient. Even if the respirator system is modular, such as that described in U.S. Patent Application Publication No. 2002/0092522 to Fabin, which is incorporated herein by reference in its entirety, the user must be equipped with several modules and must disassemble the respirator system to switch between operational modes. For example, because the exhalation units of negative pressure respirators and SCBAs have differing valve ratings, the exhalation unit must be changed when switching between modes. Not only is changing modules inconvenient, it might be impractical or impossible in situations where the air contamination is severe or especially dangerous. Hence, it is desirable to have a respirator that can quickly and easily be converted for use in various operation modes.